This invention relates to ultrasonic imaging method and apparatus and, in particular, to pulse echo method and apparatus wherein short broadband ultrasonic pulses are applied to the object under investigation, travel into the object, and are reflected by boundaries and discontinuities therein to be returned to the transducer. The received echo signals which are derived from different depths within the object are supplied to a suitable display, and the transducer is moved relative to the object to provide for a two-dimensional display. The above-described technique, generally referred to as B-scan, often utilizes a cathode ray tube display in which one of the deflection voltages is proportional to the transducer position, the orthogonal deflection voltage is proportional to the time elapsed since the energizing pulse, and the cathode ray tube beam is modulated by the received pulse intensity. The resulting image is of a section of the object lying in the plane of the propagating ultrasonic waves. A narrow beam is employed which often is focused at an operating depth within the object field for improved lateral resolution.
Such pulse echo method and means of ultrasonic imaging often are employed for imaging of living organisms. Pulses that are reflected from scatterers further within the organism experience greater attenuation, and it is common practice to compensate for such difference in attenuation by time variable gain amplification of the received signal. It will be understood, however, that the attenuation within organs or other tissues varies also with frequency of the ultrasonic wave. In particular, the attenuation coefficient of tissue increases substantially linearly with frequency, with the high frequency spectral components of the returned signal being attenuated more severely than the low frequency components. Typically, the center frequency of the received signal drops in frequency with depth of penetration, first at a moderate rate, then steeply, and finally at a low rate. In brief, not only is the amplitude of the return signal time dependent, but the spectral distribution of the return energy pulse also is time/depth dependent. Prior art echographic ultrasonic imaging system and method typically include time variable gain amplifying means in the signal processing system for increased gain with range to offset the loss of signal strength caused by tissue absorption without provision to compensate for changes in the spectral distribution of the return signal with time, or depth of penetration.